A composite material composing of a mixture of a wood cellulose filler, which is produced as a filler by finely crushing a material such as wood, hardboard, pulp or hemp, and a granular olefin series plastic material of polyethylene or polypropylene (PP) is molded by extrusion molding or injection molding, and such a composite material product is widely used as a variety of parts, boards and sheets.
Physical properties, such as flexual strength, flexual modulus, impact strength and heat deformation temperature, of such a composite material can be improved by appropriately selecting a material, a particle size and a mixing amount of the wood cellulose filler, and thus, such a composite material is also used for a variety of interior parts of a car.
However, in such a composite material composing of a mixture of a wood cellulose filler and olefin series plastic, it is known that the rigidity is decreased as the water content thereof is increased, as shown in Table 1. This is because water is excessively produced from the wood cellulose filler by heat produced when the olefin series plastic material contained in the mixture is melted and kneaded and such excessive water reduces the adhesive property between the wood cellulose filler and the olefin series plastic material.
TABLE 1 ______________________________________ Physical Properties of Extrusion Molded Sheet of Wood Cellulose Flour (W.C.F)/Plastic Composite Material ______________________________________ Compositions: Standard composition* Water content of W.C.F (%): 3-5 7-8 Strength: Tensile strength (kg/mm.sup.2) 2.71 2.49 Flexual strength (kg/mm.sup.2) 4.61 4.12 Flexual modulus (kg/mm.sup.2) 443 394 ______________________________________ *Standard composition = PP:W.C.F:Recycled composite material = 2:2:1
In order not to reduce the adhesive property therebetween and in order to improve the physical properties described above, it is necessary to dry in advance the wood cellulose filler to 3-4% water content so as to keep the water content below 5% when mixing. If a recycled composite material, which includes the same components as a composite material to be produced, is used to produce the composite material, the recycled composite material is crushed in a predetermined size and the crushed recycled composite material is used as one component of the composite material. In this case, it is necessary to dry a wood cellulose filler materials included in the recycled composite material into 5% water content.
The olefin series plastic is commonly used as a matrix of the composite material. However, the olefin series plastic is chemically inert (i.e. not polarizable) and is less affinity. Thus, an essentially good adhesive property between the matrix and the wood cellulose filler can not be achieved. That is, the improvement of the physical properties of the molded product of such a composite material is limited. In order to improve the adhesive property of the olefin series plastic which is chemically inert and is less affinity, in practice, polypropylene modified by maleic anhydride having a low degree of modification by maleic anhydride and a large molecular weight is used as an additive (a rigidity improving agent) for improving the adhesive property therebetween.
By adding such a rigidity improving agent (2-7%) to a mixture of olefin series plastic and a filler consisting of glassfiber, talc, calcium carbonate and the like, a composite material having improved rigidity and heat-resisting property can be obtained, and such a rigidity improving agent is also used for a composite material consisting a wood cellulose filler.
The polypropylene modified by maleic anhydride serves to give a certain measure of polarity for the olefin series plastic which has little polarity and to increase the affinity against the filler so as to make higher the adhesive property therebetween, and as a result, the rigidity and the heat-resisting property of the composite material are improved (see Commercial Product A in Table 2 below).
The polypropylene modified by maleic anhydride used as an additive for improving the rigidity has a low degree of modification by maleic anhydride. However, the molecular weight thereof is large, and thereby it is very expensive.
As described above, in order to improve the physical properties, the wood cellulose filler should be dried to 3-4% water content, and if a recycled composite material (leavings or scraps of the composite material) is used, the wood cellulose filler contained in the recycled composite material should be dried to 5% water content. Such a dry process needs a special drying process, and this causes the cost of the composite material (the material cost in the unit cost of production is high) to increase, and as a result, it is extremely limited to use it.
The polypropylene modified by maleic anhydride having a low degree of modification by maleic anhydride serves to improve the rigidity. However, its large molecular weight influences the elastic fluidity of a melt thereof. That is, the extrusion molding ability of a composite material including it is much lower than that of a composite material which does not include it, so that a thickness of the molded product can not be uniform and it is difficult to adjust the thickness.
Other than polypropylene modified by maleic anhydride described above, polypropylene modified by maleic anhydride having a relatively high degree of modification by maleic anhydride and a small molecular weight is commercially available as a rigidity improving agent. The cost of this agent is about half of the rigidity improving agent described before.
However, each of physical properties such as flexual strength, flexual modulus, impact strength and heat deformation temperature of a composite material (Commercial Product B in Table 2), which is produced by adding the polypropylene modified by maleic anhydride having a high degree of modification by maleic anhydride and a small molecular weight, is worse than each of the corresponding physical properties of a composite material (Commercial Product A in Table 2), which is produced by adding the polypropylene modified by maleic anhydride having a low degree of modification by maleic anhydride and a large molecular weight. This is because the molecular weight of the former is lower than that of the later so that the adhesive property between the wood cellulose filler and the olefin series plastic is reduced by water produced from the wood cellulose filler.
Therefore, the present invention is made to solve problems described above, and an object thereof is to provide a method for producing a composite material product having high rigidity and high heat deformation temperature mainly composing of a wood cellulose filler and olefin series plastic by use of a cheap rigidity improving agent.
Another object of the present invention is to provide a method for producing such a composite material product in which a special drying process does not need for the wood cellulose filler to be mixed.
Still another object of the present invention is to provide a method for producing such a composite material product in which the elastic fluidity in the molding process is good to mold into a predetermined product.